Pyrimidine derivatives are synthesized to possess various pharmacological and biological activities. In the present work, we have synthesized 3-(4-methoxyphenyl)-1-phenyl prop-2-en-1-one (1) (chalcone) by using 4-methoxybenzaldehyde and acetophenone in the presence of alcoholic KOH by Claisen-Schmidt condensation reaction. After purification and characterization by physical and spectral methods, The synthesized chalcone have been converted into 6-(4-methoxyphenyl)-4- phenyl-1,6-dihydropyrimidin-2-amine (2) by treating with guanidine nitrate in the presence of alcoholic KOH. The structure (2) has been characterized by spectral analysis. The synthesized compound (2) is further reacted with ethyl 2-cyano-3, 3-bis (methylthio)acrylate in the presence of catalytic amount of potassium carbonate in DMF under reflux condition to givenovel 2- (4-methoxyphenyl)-8-(methylthio)-6-oxo-4-phenyl-1,6,9,9a-tetrahydro-2H-pyrimido[1,2-a] pyrimidine-7-carbonitrile(3) in good yields. The compound (3) possesses replaceable methylthio (-SCH3) group at 8 position. The compound (3) is further reacted with various nucleophiles like substituted aromatic amines, aromatic phenols, hetryl amines and active methylene compounds to give 2-(4-methoxyphenyl)-8-substituted-6-oxo-4-phenyl-1,6,9,9a-tetrahydro-2H-pyrimido[1,2-a]pyrimidine-7-carbonitrile in good yields.